1. Field of the Invention
This invention relates to methods and apparatus for sputtering material on a substrate, and more particularly to planar magnetron sputtering methods and apparatus.
2. Description of the Related Art
A sputtering process to deposit thin films of various conductive or insulating material on the surfaces of objects or substrates is well-known. The target material to be sputtered and deposited on the substrate is subjected to bombardment by ions which dislodge and eject material from the target onto a substrate. The target and substrate are generally placed in a evacuable chamber containing a heavy inert gas, such as argon, maintained at a relatively low pressure. Ions are created and accelerated to high velocities in directions substantially perpendicular to the front surface of the target.
Glow discharge sputtering produces a high density of ionized bombarding particles by mounting the target on a cathode biased at a strong negative potential. The cathode and target, together with the apparatus maintained at ground potential, create an electric field substantially perpendicular to the exposed front surface of the target, and accelerate positive ions to bombard the target material which eject target atoms that subsequently condense to form a thin film on the surface of the substrate.
Sputtering efficiency may be improved by maximizing the number of ions bombarding the target surface. Planar magnetron sputtering devices utilize magnets in proximity to the target to produce a magnetic field which traps and directs electrons near the target, thereby increasing the rate of ion generation and resulting ion density near the target The electric field E, created by the cathode and ground configuration, is substantially perpendicular to the target surface, whereas the flux lines of the magnetic field B tend to exit from and return to the target surface, forming a closed arch. In the locations where the electric field E and magnetic field B are perpendicular to each other, free electrons acquire a velocity parallel to the target surface, expressed as the vector cross product of the electric field E and magnetic field B (i.e. the E.times.B velocity).
The magnet elements may be configured to restrain electron movement to a closed ring-shaped pathway parallel to the target surface. The erosion pattern of the target, resulting from displacement of the target material by the ionized gas, corresponds to the electron path and takes the form of an annular depression. Target replacement generally must occur when the annular erosion has progressed deeply into the target surface.
The current planar magnetron sputtering process is associated with several drawbacks. First, the existing devices are inefficient when operated at lower vacuum pressures, i.e. less than one millitorr. Tests conducted tinder low pressure have resulted in unstable sputtering plasma, and normally require higher voltage to operate properly. Second, excessive heating of the target results from continual bombardment by high energy ions as well as exposure to high power necessary to establish a negative potential. Therefore, effective cooling of the target material is essential. Finally, current magnetic configurations apply thin films unevenly and tend to erode the target surface inefficiently. Replacement is often required when only 20% to 30% of the target material has been utilized.
Several types of magnetron devices have been proposed to overcome some of the deficiencies associated with the existing art. For example, U.S. Pat. No. 5,262,028 to Manley discloses an apparatus having a plurality of magnets, including a center booster magnet, and a thicker target in which a center section has been cut away. This arrangement is intended to shape the magnetic field to create a wider and more uniform erosion pattern. However, distortion of the magnetic field through magnets of differing field strengths and polarities may increase target erosion efficiency at the expense of uneven thin film deposition. Furthermore, the magnetic flux lines may be dependent on target placement and be subject to change as the target is eroded. The use of a plurality of magnets to create the magnetic nodes necessary to bring the magnetic flux lines parallel to the target material may complicate the design and manufacture of the magnetron. Finally, the complicated magnet assembly is interposed between the target and cooling system, thereby interfering with temperature regulation of the target material.
Therefore, there is a need for a planar magnetron sputtering device which provides improved film quality and deposition uniformity, operates at low pressure and low voltages, facilitates effective target cooling, and is simple to fabricate.